The answer: There's a very simple trick. Basically, pick up a bunch of free weights with one hand until you find the heaviest one that you can still, well, pick up and do a move with. (So, one bicep curl, if that's what you're going for. Or one chest press. Whatever move you're hoping to practice with). "Once you know the heaviest one you can lift, you'll want to choose a weight for your actual reps that's around 60 to 70 percent of the weight of your heavy one," says Justice.

"I'm an advocate of the 8-15 rep sets, as they help develop a combo of strength and muscular endurance," says Justice. "When you go over the 20-rep range, you hit a point of diminished returns"—especially if your form is flailing.